While not limited thereto, the present invention is particularly adapted for use in the production of parts for hysteresis torque producing devices. Such hysteresis torque producing devices ordinarily utilize a rotating magnetic field to drive a rotor through its hysteresis loop. Hysteresis devices are synchronous, providing the maximum torque is not exceeded. Beyond this point, the torque transmitted is independent of the slip speed, and remains essentially constant.
The rotor for hysteresis devices, also called a follower, inherently operates at small air gaps which result in thin-walled tubular magnets, precision ground internally and externally. These rotors are unique in their magnetic design requirements since the area of the hysteresis loop is the important parameter rather than the more familiar residual induction (BR) maximum energy product (BH max) or coercive force (Hc) although these latter parameters, taken together, do approximate the area of the loop. That is, the greater the parameters, the greater is the area encompassed by the loop. Furthermore, this hysteresis area is not that of the saturation hysteresis loop, but the one corresponding to the peak field developed by a stator winding or other permanent magnet. Finally, the whole thickness of the follower does not necessarily become magnetized to the same degree.
As a result, the magnetic properties of rotor materials for hysteresis devices are very specific; and since the correlation between magnetic properties and torque transmission is not linear, the allowable range of magnetic properties is limited.
Materials presently used for hysteresis devices can generally be classified as steels, Alnico alloys, and others. Alloy steels using cobalt have found the widest use in hysteresis devices. By convention, 3%, 17% and 36% cobalt steels are standard compositions for hysteresis devices and have provided a useful choice of magnetic properties. Cobalt steels also contain amounts of tungsten, manganese and chromium, but no nickel. High carbon steel and some chromium steels are also used. These materials are usually quenched-hardened in air, oil or water to develop coercive force and, consequently, hysteresis area. Quenching demands careful attention to detail and does not necessarily produce uniform magnetic properties. Furthermore, quenching physically distorts the shapes formed from these alloys; and because of the precision dimensions required, parts formed from steel or alloys of this type are machined to a definite size before heat treatment and precision ground to final size thereafter. This is an expensive procedure with a final part having a wide range of magnetic properties frequently demanding selective assembly into a hysteresis device.
Alnico II, V or VI is used in some hysteresis devices and can produce improved volumetric efficiency (i.e., larger area encompassed by the hysteresis curve). However, to do so requires a high input power with consequent heating which must be considered. This high input power requirement has limited the use of Alnico. Some investigators have proposed alloys for hysteresis devices based on small changes in Alnico chemistry coupled with a special heat treatment. Generally, these alloys have been impossible to control. That is, the first heat treatment yields a small number of parts with the desired magnetic properties; and repeated tries each yield the same small number of parts. The result has been very expensive magnets and a process that defies scheduling. Other materials used for hysteresis devices are Cunife, Cunico, Vicalloy and P-6 (Trademarks). P-6 is probably the most widely used and is described in U.S. Pat. No. 2,596,705. This alloy is similar to Vicalloy in that it utilizes large amounts of cobalt (40% or greater). However, it requires a severe cross-sectional area reduction which produces a preferred orientation to the resulting wire or strip and a detail heat treatment. In general, it can be said that presently-used alloys are disadvantageous for the reason that they require large amounts of cobalt (i.e., 17-40% by weight), have magnetic properties which are difficult to control, result in yield of only 10-35%, and require cumbersome magnetic testing of parts since the total hysteresis area must be evaluated.